1. Field of the Invention
The present invention relates to an optical element, an optical system including the optical element, and an optical apparatus including the optical system, and relates more particularly to optical characteristics of the optical element.
2. Description of the Related Art
When a beam of light impinges on the surface of a transparent material, such as glass, a fraction of the incident light is scattered (reflected) in all directions. The surface roughness of the material is responsible for this loss of energy. Accordingly, a member used in an optical part typically has an antireflection function. A transparent member without an antireflection function has a transmittance that is decreased by about 4% to 8% per surface. Therefore, in a system that uses a plurality of transparent members without an antireflection function, for example, in a photography optical system, the quantity of transmitting light can be markedly decreased.
In order to improve the transmittance of light in transparent optical members, various methods have been suggested to apply an antireflection function. For example, Japanese Examined Patent Application Publication No. 61-51283 suggests a method of applying an antireflection film on a surface of a transparent member. A typical method of forming an antireflection film may be a method of using a film deposition device and forming a film by, for example, vapor deposition or sputtering. However, when such a film deposition device is used, a material for film deposition is limited because with some materials it is difficult to obtain a thin film having a desirable refractive index. Hence, a method has been suggested, in which a thin film having a high refractive index and a thin film having a low refractive index are selectively introduced to properly determine the thickness of a film. With this method, a film virtually having a middle refractive index can be obtained.
Alternatively, an antireflection surface structure (“antireflection structure member”) has also been suggested. The antireflection structure member uses a fine structured portion that is smaller than the wavelength of light to be used. One example of this concept is known as the “moth-eye” structure. The surface of the eye of a moth has a very low reflectivity because the eye of a moth has a particular microstructure. Specifically, the cornea surface of a moth's eye is seen to have microscopic raised protuberances in the sub-wavelength range. This sub-wavelength surface-relief profile constitutes a very low reflectance interface for light and appears to absorb light from any direction. Therefore, it has been determined that when light interacts with finely structured portions that are smaller than the wavelength of light, the light does not recognize the structure portions as such and behaves as if the structure portions are formed of a uniform medium. Then, the structure member apparently has a refractive index based on a volume ratio of a material that forms the structure portions. Using this concept, a structure member having a low refractive index, which is not obtained with a normal material, can be provided. Since the material having the low refractive index is used, an antireflection function with higher performance can be obtained.
Japanese Patent Application Laid-Open No. 2005-62674 suggests an antireflection structure member using the above-described microstructure concept. The antireflection structure member includes structure portions each having a shape that is tapered toward the surface. With this shape, it is suggested that a reduced refractive index is gradually decreased from the substrate side toward the surface side.
Japanese Patent Application Laid-Open No. 2003-240904 defines the shape of protrusions of a microstructure. When the shape of a most protruding portion of the protrusions is compared with the shape of a most recessed portion thereof, the shape of the most protruding portion is more tapered. Thus, a change in refractive index at the boundary between the structure and a substrate is decreased, and reflection is decreased.
However, with the concept suggested by Japanese Examined Patent Application Publication No. 61-51283, since the antireflection film uses the material having the high refractive index, the antireflection film may have degraded wide-band characteristics. With the suggestion by Japanese Patent Application Laid-Open No. 2005-62674, although the microstructure is tapered to apply inclination to the refractive index, the document does not disclose a structure having an optimum refractive index. In addition, the document does not disclose a configuration regarding facilitation of fabrication. With the suggestion by Japanese Patent Application Laid-Open No. 2003-240904, an attention is paid only on the change in refractive index at the interface. Thus, a portion with a large change in refractive index may appear, resulting in difficulty of obtaining wide-band characteristics.